/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseBehaviors/InputBehaviorModifierStates.h
Register a modifier ID and an associated test function
void RegisterModifier ( int ModifierID, const [FModifierTestFunction](API\Runtime\InteractiveToolsFramework\BaseBehaviors\FInputBehaviorModifierStates\FModifierTestFun-) & ModifierTest )